side of the road - again

pcan, how long does it sit before it will start back up?

Well, when my scooter first started exhibiting those stop and go like symptoms, I found that the fuel pump was not pumping enough fuel to keep the carb's float bowl full when under high loads or high speed operation. So I replaced the pulse pump with an electric fuel pump.

Basically the engines are set to run extra lean from the factory. They choose a main jet that lets the engine run OK, extra lean with the float bowl fuel level a little lower than optimum, so as to compensate for the fuel pump not pumping all that well. But as fuel pump diaphragms and one way rubber valves deteriorate and get harder, the fuel level drops lower and the engine runs a little more leanly. Eventually the engine runs too leanly and stalls out from fuel starvation. But for a few seconds as the engine slows down and stops the fuel pump is still pumping and the carb's float bowl gets a little dribble of fuel in it. Then later after sitting for a while, you go to start the engine, it fires up as you have the throttle set to idle and you crnak it over, the engine fires right up, and the carb's float bowl fills up with fuel like normal. The process repeats itself ad nauseum.

I decided to go with a small electric fuel pump and not bother with the pulse pumps. Pulse pumps are more effective on a 2 cycle engine where they get much stronger pressure pulses off the crankcase. 4 cycle engines use the intake manifold pressure pulses, of which as the engine runs faster and faster, the pressure pulses get weaker and weaker. When I did tests, the fuel pressure was at its maximum at just off of idle, and steadily decreased as the engine speed when up or the engine was under a load (like climbing a hill). Maximum fuel pressure was like 1.25 to 1.5 psi. Crusing at 40 mph had like .5 psi.

Anyway a electric fuel pump keeps the fuel pressure constant, plus a side benefit was the engine ran more rich at higher speeds. So it was like putting in a larger main jet size on the carb. So the engine was happier and didn't run as hot due to the lean burn effect of running extra lean.

Things that effect the pulse pump. Deteriorating diaphragm or rubber one way valves cause the fuel pump to not work well.
Intake and exhaust valve gaps closing up, causes the pressure pulses in the intake manifold to be weaker.
Vacuum lines cracked or leaking, or too soft of a vacuum line flexing with the pressure pulses, thus reducing what the pulse pump gets.
A cracked or leaking rubber intake manifold.

Here is a thread with my testing on fuel pumps here:
www.powerscooters.org/cgi-bin/discuss/YaBB.pl?num=1211083553

Hi pcan,

Earlwb's explanation along with your fuel delivery description would probably be the reason for the high speed shut down. There maybe a couple of problems because of the complaint about hard starting when cold. The carb could be low on fuel at cold startup which could also be a problem due to low fuel delivery or possibly the auto enricher is sticking with the needle extended and heating the carb slightly could allow the needle to retract and allow the engine to start. JRR also has a point with the CDI overheating. The hair dryer thing would help pinpoint that too. CDI overheating of course will not cause the cold starting problem.

66

I think Earl is right. Any malfunctioning item sometimes works good when cold and then works worse as it heats up. I'd change to one of those DF-44 Mikuni pumps or whatever is on a Honda Reflex or you get the idea. It sounds like a defective pump to me.

Well, it's a gravity feed for sure. Part number 12 in the diagram is the standard vacuum fuel petcock.



66

Knowing it is gravity feed makes all the difference. First I would bypass the vacuum petcock, some member had to do that, see if the problem is still present. Here is what I don't understand, when the engine quits, the petcock will shut off. I guess after it sits a while the fuel in the fuel line could slowly trickle back into the Carb bowl, finally allowing enough fuel to let it re-start. I think it is a bad fuel petcock , or low vacuum at higher rpm, or when hot, So this little blurb gets us back where we started. It will be a process of elimination, but all can be done at home not in the shop at $60 per hr. Bypass the petcock first, Good luck

It sounds like a bad CDI. Put a Ziplock bag of crushed ice on it and tie wrap it. Take it for ride and see if it cuts out again. It probably won't until the ice melts.

jr, I guess what leads me to what I said is the fact that when he pulled the fuel line at the carb, the fuel flowed fast at idle and slowed to a trickle at higher RPM. This should have nothing to do with the Electricals and points towards vacuum. Its not a Vacuum fuel pump, so the vacuum petcock must be closing up to stop the fuel flow, Probably back to your suggestion of needing the valves adjusted , or a very small vacuum leak. Don't you just love these little Puzzles. Hey we have snow predicted for tomorrow, Glad we rode yesterday in our sunny 75 deg temp. Have a good evening Youallll .
.

This reminds me about reading of a weakness in the original Helix engines which would lead to a vacuum leak. The weight of the carburetor was supported only by the intake manifold which would eventually lead to cracking and a vacuum leak. Honda does have a carb support kit to correct the problem. Looking at the CfMoto layout seems to show some type of carb bracket. Checking the intake manifold for tightness and cracks with a spray of carb cleaner would be a good idea. If the engine speed changes when spraying the carb cleaner in this area, then there is a vacuum leak.

66

Make sure your air intake box is secured rightly. If there are any leaks, it will get more air than it should, run lean and cut out at higher RPM's. Case in point, just by taking the cover off my intake box to get to the filter, that alone adds so much extra air that it won't rev up past 4.5k rpm.